Definitely a light leak. For what it's worth, this page on the Zenit B (first hit I got on google) confirms that this camera make is prone to light leaks, but suggests that they're relatively easy to repair.
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mattdmJan 15 '12 at 20:15

The facts that it has different brightness on different photos and is quite bright on an otherwise dark frame suggest the leak is independent of exposure. Being in the upper edge, I'd suspect lower edge of the body.
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ImreJan 15 '12 at 21:12

1

I'd agree, definitely a light leak along the lower edge. I guess thats not too surprising for 40 year old camera that was known for light leaks. ;)
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jrista♦Jan 15 '12 at 23:29

You could wrap the camera in several layers of black plastic (from thick garbage bags) with only the front of the lens visible, this would confirm it is a light leak. By then pulling back the amount of camera covered will easily help you to find where the leak is.
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Barry SempleJan 16 '12 at 2:05

From the sample shots, the amount of flare varies with shutter speed. In the first shot, where it's a major blowout, the aperture was small, suggesting a longer exposure. In the scenic, where the exposure was short, the effect is much smaller. That would point to something in front of the shutter. If the leak were on the camera back, all of the frames would show completely blown out highlights because the leak would be continuous.

I would look closely at the foam under the viewfinder screen. A very small bit of lens damage is a possibility, because the pattern suggests multiple internal reflections, but not something on the front element. Given that the Zenit has a threaded lens mount, the mount itself is an unlikely suspect.